During the growth in popularity of decorative appliques on garments such as T-shirts or jackets, there has been a continuing desire for ways to make such appliques retroreflective. Retroreflective appliques have the ability to return a substantial portion of incident light in the direction from which the light originated. On an outer garment worn at night, such retroreflective appliques provide a bright return of light to oncoming motorists, thereby adding a safety feature, as well as increased decorative appeal, to the garments.
A retroreflective applique typically includes a layer of optical lens elements, a polymeric binder layer, and a specular reflective layer. The optical lens elements usually are transparent microspheres that are partially embedded in the polymeric binder layer such that a substantial portion of each microsphere protrudes from the polymeric binder layer. The specular reflective layer is disposed on the portion of the transparent microsphere that is embedded in the polymeric binder layer and typically comprises aluminum, silver, or a dielectric mirror. Light striking the front surface of the retroreflective applique passes through the transparent microspheres, is reflected by the specular reflective layer, and passes back through the transparent microspheres to travel back in the direction of the light source.
Several methods are known or have been proposed to provide such retroreflective appliques. One such method involves applying a layer of glass microspheres over a printed design; however, this layered product is stiff and unsuitable for conformable garments. Another method involves silk-screening a design onto a garment, and then while the design is still wet, cascading microspheres onto the design; however, such an approach is messy, usually provides a nonuniform deposit of microspheres, and is impractical for obtaining high reflective brightness. Yet another method involves mixing hemispherically specularly coated glass microspheres into ink and printing this onto the garment; however, this results in a reduced retroreflectance because the hemispherically coated microspheres are randomly oriented within an applied coating.
In addition to being conformable or drapable so they can be used on garments, retroreflective appliques should be able to withstand laundering conditions. Examples of launderable retroreflective appliques are disclosed in U.S. Pat. No. 4,763,985 (Bingham), U.S. Pat. No. 5,200,262 (Li), and U.S. Pat. No. 5,283,101 (Li). These appliques, however, are not imagewise printed appliques, nor are they prepared using compositions having a viscosity or volatility that are suitable for imagewise printing, especially by screen printing. Furthermore, retroreflective appliques having multicolored designs are particularly desirable. Examples of multicolored retroreflective appliques are disclosed in U.S. Pat. No. 4,102,562 (Harper et al.) and U.S. Pat. No. 5,344,705 (Olsen), and PCT Application No. WO 92/07990 (Olsen et al.). Retroreflective appliques that can be multicolored, conformable and drapable, and exhibit very little loss of retroreflective brightness after a significant number of washings, are extremely desirable.
U.S. Pat. No. 5,344,705 (Olsen) and PCT Application No. WO 92/07990 (Olsen et al.) disclose a retroreflective transfer sheet material comprising a base sheet; a continuous layer of transparent microspheres partially embedded in a heat-softenable layer on the base sheet; a two-part resin composition comprising a polyester and an isocyanate hardener printed on the microsphere layer in an imagewise pattern; a two-part resin composition comprising a polyurethane or a polyester, an isocyanate hardener, and reflective flakes printed directly on the microspheres or on the polyurethane-based resin; and a two-part extender resin comprising a polyurethane or a polyester and an isocyanate hardener printed in an imagewise pattern. Before the extender resin is hardened, a powdered hot melt adhesive is applied to the wet image. This powdered hot melt adhesive must be fused into the extender resin through the application of heat. After the image dries, the powdered hot melt adhesive is removed from the nonimage areas. This transfer sheet material is used by laying it on a substrate, with the transfer layer against the substrate, adhering the transfer layer to the substrate, and then stripping the base sheet and heat-softenable layer away.
The resulting transferred applique shows good home wash durability; however, the image is generally stiff and hard. This attribute leads to cracking when the fabric having the applique thereon is washed or flexed. These cracks serve as erosion and abrasion sites for further image and brightness degradation during wear and laundering. If appliques made with these systems are applied to a substrate fabric that stretches, the image will break or crack open, leaving islands of the now broken image that move with the stretched fabric. When the fabric is allowed to return to its rest position, the islands of image do not completely join to reform an uninterrupted image. The cracks between image pieces remain open and are sites where wash erosion and wear abrasion can start.
U.S. Pat. No. 4,102,562 (Harper et al.) discloses appliques that are softer and more flexible than the appliques of Olsen; however, they do not wash well. The Harper et al. transfer sheet comprises a base sheet; a continuous layer of transparent microspheres partially embedded in a heat-softenable layer on the base sheet; a specular reflective layer (typically, a dielectric mirror) deposited onto the exposed surfaces of the microspheres; and a thick transfer layer (typically, a color layer) of a vinyl plastisol ink printed over the microsphere layer in an imagewise pattern. An adhesion-promoting layer comprising a silane is coated on the microspheres prior to the vinyl plastisol ink being coated thereon. This adhesion-promoting layer includes either a glycidoxypropyltrimethoxysilane or a polyurethane and a silane having reactivity toward polyurethane. Although Harper et al. disclose that they believe the silane provides a chemical linkage between the vinyl plastisol ink and the glass beads, the applique does not retain its retroreflective brightness because the microspheres tend to fall off after only a few washings. In fact, Harper et at. disclose that after only five machine washings, the appliques on sample garments displayed a 60% loss in retroreflectivity.
Thus, what is needed is a retroreflective applique that is softer, more durable, and easier to manufacture than conventional retroreflective appliques and transfer sheets for applying such appliques to a substrate, such as an article of clothing, for example.